Heretofore, there is a display apparatus provided with a display device which effects color display depending on three types of signals corresponding to images of three colors of red, green and blue. As such a display apparatus, there are various display devices such as CRT, plasma display (PDP), organic EL display (OLED), and these display apparatuses have already been widely put into practical use. Among them, the LCD has characteristics, such as a thin shape, a low power consumption, and a high display quality, so that it is applied to every color display apparatus, such as a cellular phone, a monitor for PC, a home television, and the like.
Almost all the color display methods of the LCD employ a microcolor filter (MCF) method wherein a liquid crystal display device capable of performing monochromatic modulation is used and one pixel is divided into three subpixels which are provided with color filters of red, green and blue, respectively. As a color display method different from this, a field sequential color (FSC) method wherein a display state of a liquid crystal display device capable of performing monochromatic modulation is switched at high speed and light sources of red, green and blue are synchronized with the display device, thereby to utilize a color mixture effect of the three primary colors by time division.
On the other hand, in the conventional LCDs, there are those with no color filter and there have been known a liquid crystal display apparatus of ECB-type (electrically controlled birefringence (effect)-type) proposed in U.S. Pat. No. 6,014,195 or Japanese Laid-Open Patent Application No. Tokkaihei 6-175125.
In the case of the liquid crystal display device of a transmission-type, linearly polarized light which comes in through one of polarization plates is changed into elliptically polarized light consisting of respective wavelength light fluxes different in state of polarization by the action of birefringence of liquid crystal layer in a process of transmitting a liquid crystal cell. The elliptically polarized light enters the other polarization plate and the transmitted light having passed through the other polarization plate is colored light consisting of light fluxes of colors corresponding to light intensities of the respective wavelength light fluxes.
More specifically, the ECB-type liquid crystal display apparatus (hereinafter, referred to as an ECB color LCD) is capable of coloring light by utilizing the birefringence action of the liquid crystal layer of the liquid crystal cell and the polarization action of polarization plate, so that it causes no light absorption by the color filter, thus effecting bright color display at a high transmittance of light. In addition, the birefringence of the liquid crystal layer is changed depending on a voltage applied to the liquid crystal cell, so that by controlling the voltage applied to the liquid crystal cell, it is possible to change the color of the transmitted light or the reflected light. By this, it is possible to display a plurality of colors at the same pixel.
FIG. 15 is a diagram showing a relationship between an amount of birefringence (called retardation R) of liquid crystal display device used in the ECB color LCD and coordinates. According to this figure, it is found that the color at a retardation R from 0 to about 250 nm is achromatic color since the retardation range is located substantially at a center portion of the chromaticity diagram but is changed when the retardation exceeds the retardation range.
When a liquid crystal material having a dielectric anisotropy (Δε) which is negative is used as the liquid crystal and liquid crystal molecules thereof are homeotropically (vertically) aligned with respect to the substrates, the liquid crystal molecules are inclined with voltage, so that an amount of birefringence is increased with a degree of the inclination of the liquid crystal molecules.
In this case, in a cross-nicol condition, the chromaticity is changed along a curve indicated in FIG. 15. For example, when the voltage is not applied, the retardation R is substantially zero, so that light does not pass through the display device to provide a dark (black) state. With an increase in voltage, brightness (lightness) is increased in the order of black, gray, and white. When the voltage is further increased, the light is colored to change the color (hue) in the order of yellow, red, violet, blue, yellow, violet, light blue, and green.
As described above, under voltage application, the ECB color LCD at the vertical alignment mode is capable of changing the brightness between a maximum brightness and a minimum brightness in a modulation range on a low voltage side under and changing a plurality of hues in a higher voltage range.
And now, the LCD provides a high display quality even in motion picture display and has been applied to and developed into a big-screen television. Among others, an overdrive driving method (OD method) reported by H. Okumura et al., in SID '92, pp. 601-604 (1992) has been frequently used in the LCDs for motion picture display.
The OD method is used in order to improve a low halftone response speed of LCD. For example, in the LCD in a normally white mode (display mode in which white display is effected at the time of no voltage application), when a previous state of white is switched into a halftone level, a response time required for reaching an objective halftone level is shortened by setting a voltage value only in one frame immediately after the switching so as to be somewhat higher than a voltage value for displaying an original halftone level. When the previous state is black, the voltage value is set to be somewhat lower than the original voltage value. In a second frame or later, a drive voltage for displaying the original halftone level may be applied as usual.
By this method, optical response of the LCD can be completed in about one frame or within one frame.
As described above, by improving the response speed of liquid crystal in the monochromatic modulation area, it becomes possible to obtain a high-quality motion picture even in either of the MCF method or the FSC method.
Incidentally, as described above, in the conventional color LCD, although the response time is almost within one frame, it takes a time close to one frame with respect to halftone response.
When switching from white to an intermediary tone of blue in the LCD using the MCF method, all the subpixels of red, green and blue are placed in an ON state in the white state, and in the intermediary tone state of blue, the red and green subpixels are placed in an OFF state and the blue subpixel is placed in an intermediary brightness state. When response at this time is observed in detail, in a state immediately after the switching of voltage, the red and green subpixels are in a transition response state between the ON state and the OFF state and the blue subpixel is in a transition response state between the ON state of blue and the intermediary tone state of blue.
In other words, in the first frame immediately after the switching, red and green are in the respective intermediary tone states and blue is in an intermediary tone state brighter than a desired intermediary tone state after the switching. When this is viewed with eyes, it is possible to say that display of blue which is low in color purity and somewhat bright is effected.
Further, similarly, the display color is changed from black to the intermediary tone of blue, it is found that a color purity is the same as a desired one in the first frame immediately after the switching but somewhat dark blue display is effected.
As described above, even when the LCD has a response speed of about one frame period, complete display is not effected, so that in the first frame immediately after the switching, display somewhat different in brightness or color saturation is effected.
In the MCF method, as described above, the hue (or systematic color) is not largely changed, so that it is considered that looking and listening become possible without feeling large inconformity even in the case of displaying quick motion picture as in a liquid crystal television which is currently commercially available.
On the other hand, in the ECB color-type LCD, e.g., when switching from white to blue is performed, transitional response with a change in hue is observed in the first frame immediately after the switching. More specifically, in the transitional response state from white to blue, along a curved line shown in FIG. 15, intermediary display colors are represented in the order of yellow, red, and violet and thereafter blue display is effected.
In short, in the transitional response state, a display color, such as magenta, different in systematic color from blue is observed. Further, when such a display color different in systematic color is observed, coloring is observed at an edge portion of a moving body in the motion picture display to result in inconformity with respect to the motion picture display.
There is also the coloring case not only in the motion picture display but also when switching from a still picture (image) to another still picture is performed.
Further, in the case where dithering is used at the time of displaying natural picture, two-valued gradation control is effected at a unit pixel but gradation display by a spatial color mixture effect is performed, so that two-valued information to be displayed is largely changed even when an image is slightly changed. Accordingly, in the case where an image effecting natural picture display by dithering is displayed as motion picture, the change becomes a cause of destruction of color balance not only at the edge portion but also in the entire image.